1. Field of the Invention
The described invention relates to the field of integrated circuit packages. In particular, the invention relates to an integrated circuit package capable of being mounted to a circuit board via a mass reflow process.
2. Description of Related Art
A windowed integrated circuit package is used for various applications in which an integrated circuit is illuminated or irradiated by light or other radiation sources located outside the integrated circuit package. An image sensor is one use of a windowed integrated circuit package.
For example, a photodiode array may be placed within a windowed integrated circuit package. The photo-detector array provides an image data output based upon the light incident on the photo-detector array. The photo-detector array may be used for capturing images or for other image reproduction applications. A color filter array (FA) material is used with the photo-detector to filter the light impinging on the image sensor to allow for the formation of full color images. Each filter allows a predetermined color of light to reach a corresponding photo-detector, thus determining what color light will be sensed by the photo-detector. By grouping sets of light sensors together, the intensity and color of light reaching an area can be determined.
Integrated circuit (IC) packages are mounted on circuit boards by various techniques including mass reflow and manual and hot bar soldering of the package to the circuit board. Manual soldering and hot bar soldering, however, are relatively slow and expensive processes.
Mass reflow board mounting is a faster, automated process. Mass reflow refers to one of several different techniques that raise the temperature of the IC package to approximately 215 to 225° C. At these elevated temperatures, solder residing on pads of the integrated circuit board melts and adheres to leads on the IC package. After the solder cools, the IC package remains firmly coupled to the solder pads. Mass reflow includes infrared, convection, and vapor phase techniques.
Non-ceramic packages such as windowed plastic packages are more desirable than ceramic packages because they are of lower cost than corresponding ceramic windowed packages. However, until recently, standard windowed plastic packages tested on the mass reflow process exhibited problems such as cracked lids, delamination of the die from the die attach, and lid-sealant separation due to thermal expansion mismatch between the plastic and glass window. Until recently, these windowed plastic packages were mounted to circuit boards using techniques such as manual soldering that keep the package bulk from reaching the elevated temperatures of the mass reflow process.
Co-pending U.S. Patent application Ser. No. 09/172,710, entitled “Mass Reflowable Windowed Non-Ceramic Package,” assigned to Intel Corporation and Kyocera Corporation describes a windowed non-ceramic package that does meet the thermal requirements for mass reflow board mounting. However, a way of reducing internal stresses is desirable. This would allow extended bake times to be reduced or eliminated, and larger package sizes to be mass reflowed.